In CT systems, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient, and impinges upon a linear array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object. Each detector of the linear array produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
The x-ray source and the linear detector array in a CT system are rotated with a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector.
In an axial scan, signals from the detector array are processed to construct an image that corresponds to a two dimensional slice taken through the object. Such processing is sometimes referred to as image reconstruction. One image reconstruction method is referred to as the filtered back projection process. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Reducing image artifacts in CT generated images, of course, is desirable. Such artifacts are generated for different reasons such as, for example, if the attenuated X-ray beam at the detectors is weak. Such a condition is known as X-ray photon starvation. Since X-ray photons are absorbed by the human body, the strength (or magnitude) of the X-ray beam is reduced as the beam travels through the body. Accordingly, X-ray photon starvation occurs most often when the X-ray beam must travel through a lengthy region of a patient's body.
Two types of artifacts are commonly associated with X-ray photon starvation conditions. One artifact type is known as "shading" which results from CT number shifts. Such a significant difference in the CT numbers results in the image appearing darker, or shaded. The other artifact type associated with X-ray photon starvation is a severe streaking artifact which appears between two highly attenuating objects. In many cases, if X-ray photon starvation conditions are present, the artifacts are so severe that the images have to be discarded.
To reduce the occurrence of the artifacts resulting from X-ray photon starvation, CT technicians are trained to select proper X-ray source operations (kV, mA) and slice thickness for different patient sizes. In addition, patients are instructed to place their arms outside the scanning field of view (FOV) to reduce any unnecessary photon absorption by the arms.
In many cases, however, the X-ray tube specifications or other limitations prevent optimization of the X-ray source operation. Also, due to the many variations in patient anatomy, using different scanning techniques for different body types and regions can be very time consuming. Using such time consuming techniques reduces patient throughput, which is undesirable. Furthermore, even when such techniques are attempted to be practiced, some patients are uncooperative. For example, instructions such as "hold your arms over your head" may be ignored by some patients.
Adaptive filtering techniques also have been proposed to correct projection data for any X-ray photon starvation artifacts. With these techniques, a "smoothing" operation, adapted based on the X-ray photon flux in the projection, is performed. "Smoothing" operations generally involve adjusting the signal detected at one channel based on the detected signal magnitude at the channel and the magnitudes of the detected signals of adjacent channels. Such "smoothing" is performed on a channel by channel basis to eliminate shading and streaking type artifacts. Since only a small percent of projection readings experiences X-ray photon starvation, the impact of such smoothing operations on system resolution is minimal. Executing such smoothing operations, however, requires extensive computation. As a result, adaptive filtering techniques have not been widely used since implementing such techniques increases system costs.
There exists a need to cost effectively eliminate artifacts generated due to photon starvation conditions. Eliminating such artifacts, however, should not require using different scanning techniques for different body regions nor rely upon patient cooperation.